212 



ON TIDES AND TIDAL ACTION IN HARBORS. 



is called tlie diurnal inequality; it reaches its maxiDium wbeu the moon 

 is at its greatest northern or sonthern declination, and disappears when 

 it is on the e(iuator, and conseqnently has a half -monthly period. The 

 variations of height from this canse prodnce a corresponding inequality 

 in the times of high water. The sun's declination affects the tides in a 

 similar manner, but the amount of the disturbance is very small, and its 

 period extends over half a year. In long series of observations its efl'ect 

 is nevertheless well marked, both in height and time. The diurnal in- 

 equality, depending upon the moon's declination, is, on the other hand, 

 quite sensible, and in many places constitutes a prominent feature of 

 the tides, as on the Pacific coast of North America. 



DIURNAL INEQUALITY 



PARALLACTIC INEQUALITY. 



A further cause in the variation of the height of the tides is the varia- 

 tion of the distances of the sim and the moon by reason of the ellipticity 



